Catalysts and processes for the manufacture of vinyl acetate

ABSTRACT

A shell impregnated catalyst for use in the production of vinyl acetate from ethylene, acetic acid and an oxygen containing gas is provided. The catalyst has a productivity of greater than 661 grams of vinyl acetate per hour per liter of catalyst at 150° C. and consists essentially of: 
     (1) a catalyst support having a particle diameter from about 3 to about 7 mm and a pore volume of 0.2 to 1.5 ml per gram 
     (2) palladium and gold distributed in the outermost 1.0 mm thick layer of the catalyst support particles, and 
     (3) from about 3.5 to 9.5% by weight of potassium acetate. 
     The catalyst is characterised by having a gold to palladium weight ratio in the range 0.60 to 1.25.

This application is a division of application Ser. No. 07/696,215, filedMay 6, 1991, now U.S. Pat. No. 5,185,308.

The present invention relates to improved palladium/gold catalystsuseful in effecting the production of vinyl acetate from ethylene,acetic acid and an oxygen containing gas.

The production of vinyl acetate by reacting ethylene, acetic acid andoxygen together in the gas-phase in the presence of a catalystcontaining palladium, gold and an alkali metal acetate promoter isknown. The catalyst components are typically supported on a porouscarrier material such as silica or alumina.

In early examples of these catalysts, both the palladium and gold weredistributed more or less uniformly throughout the carrier (see forexample U.S. Pat. Nos. 3,725,680, 3,743,607 and GB 1333449). This wassubsequently recognised to be a disadvantage since it was found that thematerial within the inner part of the carrier did not contribute to thereaction since the reactants did not diffuse significantly into thecarrier before reaction occurred. In other words, a significant amountof the palladium and gold never came into contact with the reactants.

In order to overcome this problem, new methods of catalyst manufacturewere devised with the aim of producing catalysts in which the activecomponents were concentrated in the outermost shell of the support(shell impregnated catalysts). For example GB 1500167 claims catalystsin which at least 90% of the palladium and gold is distributed in thatpart of the carrier particle which is not more than 30% of the particleradius from the surface, whist GB 1283737 teaches that the degree ofpenetration into the porous carrier can be controlled by pretreating theporous carrier with an alkaline solution of for example sodium carbonateor sodium hydroxide.

Another approach which has been found to produce particularly activecatalysts is described in U.S. Pat. No. 4,048,096. In this patent shellimpregnated catalysts are produced by a process comprising the steps of(1) impregnating a carrier with aqueous solutions of water-solublepalladium and gold compounds, the total volume of the solutions being 95to 100% of the absorptive capacity of the catalyst support, (2)precipitating water-insoluble palladium and gold compounds on thecarrier by soaking the impregnated carrier in a solution of an alkalimetal silicate, the amount of alkali metal silicate being such that,after the alkali metal silicate has been in contact with the carrier for12 to 24 hours, the pH of the solution is from 6.5 to 9.5; (3)converting the water soluble palladium and gold compounds into palladiumand gold metal by treatment with a reducing agent; (4) washing withwater; (5) contacting the catalyst with alkali metal acetate and (6)drying the catalyst. Using this method, catalysts having a specificactivity of at least 83 grams of vinyl acetate per gram of preciousmetal per hour measured at 150° C. can, if it is alleged, be obtained.

At column 5 of U.S. Pat. No. 4,048,096 it is taught that the processdescribed above is preferably used to prepare catalysts containing 1.65to 3.3 grams of palladium and 0.75 to 1.5 grams of gold per liter offinished catalyst. For a typical silica support having a density of say600 grams per liter, these ranges correspond, in % wt. terms, to 0.25 to0.5% and 0.12 to 0.22% and hence to a Au:Pd % wt. ratio of from 0.16 to0.75. The amounts of alkali metal acetate taught as being effective are5 to 60 grams per liter preferably 25 to 35 grams per litercorresponding, in the case of potassium acetate, to 0.75 to 9.2% byweight preferably 3.8 to 5.4%.

Six examples according to the invention are disclosed in U.S. Pat. No.4,048,096. All of these teach the use of catalysts having a gold topalladium weight ratio in the range 0.42 to 0.45 and a gold content of2.1 grams per liter or less. The most active catalyst described is thatshown in Example 3 which has a gold content of 2.1 grams per liter, agold to palladium weight ratio of 0.42 and a productivity of 610 gramsof vinyl acetate per hour per liter of catalyst at 150° C. Computerpredictions of the activity of this catalyst on the basis of data wehave obtained during our studies indicates a higher activity at 661.

It has now been found that shell impregnated catalysts having aproductivity in excess of 661 grams of vinyl acetate per hour per literof catalyst at 150° C. can be obtained by ensuring that the weight ratioof gold to palladium is in the range 0.60 to 1.25. Furthermore it hasbeen found that for a given metals loading the productivity can befurther improved by ensuring that the potassium acetate content of thecatalyst is in the range 3.5 to 9.5% by weight.

This finding is quite unexpected and contrary to the teaching of U.S.Pat. No. 4,048,096 which suggests that a gold to palladium weight ratioof less than 0.5 should be used.

Shell impregnated catalysts are also disclosed in U.S. Pat. No.4,087,622. At column 2, lines 47 to 50 it is taught that the percentageof gold metal relative to the combined weight of palladium and goldmetals in preferably from about 5 to 60 percent by weight. Such figurescorrespond to a broad range of gold to palladium weight ratios of 0.05to 1.5 indicating that no significance was attached to the criticalityof this variable. Furthermore a broad range of alkali metal acetatecontents (1 to 30 weight %) is taught and only catalysts havingpotassium acetate contents of 3% are exemplified. Examples 4-1 to 4-7show the effect of varying the gold content of the catalyst relative toa fixed palladium loading but there is nothing to suggest thecriticality of the gold to palladium ratio over a wide range ofpalladium loadings.

According to the present invention there is provided a shell impregnatedcatalyst for use in the production of vinyl acetate from ethylene,acetic acid and an oxygen containing gas, said catalyst having aproductivity of greater than 661 grams of vinyl acetate per hour perliter of catalyst at 150° C. and consisting essentially of:

(1) a catalyst support having a particle diameter from about 3 to about7 mm and a pore volume of 0.2 to 1.5 ml per gram

(2) palladium and gold distributed in the outermost 1.0 mm thick layerof the catalyst support particles, and

(3) from about 3.5 to about 9.5% by weight of potassium acetate whereinthe gold to palladium weight ratio in said catalyst is in the range 0.60to 1.25.

The catalysts of the present invention have the additional advantagethat there are highly selective towards the production of vinyl acetateat the expense of by-products such as carbon dioxide.

Turning to the components of the catalyst it is preferred that thecatalyst support is either a porous silica, alumina silica/alumina ortitania with the former being most preferred. The support should have asurface area in the range 100-800 m² per g. As regards the palladiumcontent of the catalyst this is suitably in excess of 2.5 grams perliter of catalyst in order to obtain the high productivities referred toabove. For typical catalysts having a sodium content of about 0.5% byweight the palladium content should preferably be greater than 3.0 gramsper liter of catalyst, more preferably greater than 3.9 grams per literof catalyst and most preferably in the range 3.9 to 6.1 grams per literof catalyst. The gold content on the other hand should suitably begreater than 1.5 grams per liter of catalyst and for catalysts havingthe above-mentioned sodium content preferably greater than 1.8 grams perliter of catalyst, more preferably greater than 2.3 grams per liter ofcatalyst and most preferably in the range 2.3 to 7.6 grams per liter ofcatalyst. Finally it is preferred that the gold to palladium weightratio is in the range 0.7 to 1.05 most preferably 0.85 to 1.05.

Turning to the potassium acetate content of the catalyst it is preferredthat this is in the range 5 to 9% by weight to obtain optimum activity.

In an embodiment of the present invention, it has been found that thelevel of undesirable ethyl acetate byproduct can be substantiallyreduced by employing catalysts in which the gold to palladium weightratio is greater than or equal to about 0.9.

The catalysts of the present invention may conveniently be prepared bythe method described in detail in GB 1559540. In the first stage of thisprocess, the support is impregnated with a solution containing therequired amounts of palladium and gold in the form of soluble salts.Examples of salts include the soluble halide derivatives. Theimpregnating solution is preferably an aqueous solution and the volumeof solution used is such that is corresponds to between 95 and 100% ofthe pore volume of the support preferably 98-99%.

After impregnation the wet support is treated with an aqueous solutionof an alkali metal salt selected from alkali metal silicates, carbonatesor hydroxides. The amount of alkali metal salt used is such that afterthe solution has been in contact with the impregnated support forbetween 12 and 24 hours, the pH of the solution is suitably in the range6.5 to 9.5 preferably 7.5 to 8 when measured at 25° C. Preferred alkalimetal salts are sodium metal silicate, sodium carbonate and sodiumhydroxide.

During the treatment described above palladium and gold hydroxides arebelieved to be precipitated or incorporated onto the support. To convertsuch materials into the metallic state the impregnated support istreated with a reducing agent such as ethylene, hydrazine, formaldehydeor hydrogen. If hydrogen is used it will usually be necessary to heatthe catalyst to 100°-300° C. in order to effect complete reduction.

After the steps described above have been carried out, the reducedcatalyst is washed with water, impregnated with the required amount ofalkali metal acetate and thereafter dried.

The catalysts of the present invention when prepared by theabove-mentioned process typically contain 0.5% by weight sodium derivedlargely from the precipitating agent. As will be described in detail ina further separate patent application it is desirable that the sodiumcontent of such catalysts is less than this figure in order to obtainoptimum catalyst productivity. The sodium content of the catalyst can bereduced for example by washing with potassium acetate solution or by theuse of non-sodium containing reagents.

Preparation of vinyl acetate using the catalysts of the presentinvention is typically effected by contacting ethylene, acetic acid andoxygen or air with a sample of the catalyst at a temperature in therange 100° to 200° C., preferably in the range 140° to 180° C., and atpressure in the range atmospheric to 20 bars. Typically the process iscarried out heterogeneously with the reactants being present in the gasphase and with levels of oxygen below the limits of flammability. Thereaction is usually carried out with an excess of ethylene whilst theamount of acetic acid is determined by dew point considerations. Afterreaction, the vinyl acetate is separated and purified using conventionalmethods.

The present invention will now be illustrated with reference to thefollowing Examples.

General Method for Preparing the Catalyst Samples

During this method only de-ionised water was used.

Step (1) Impregnation of the Support

15 grams of a high surface area spherical silica support (KA160-ex SudChemie) was added to 8.7 ml of an aqueous solution of Na₂ PdCl₄ andHAuCl₄. The amounts of the palladium and gold complexes used were suchas to achieve the desired palladium and gold loadings on the support.The addition was done in a single portion and the mixture was gentlyswirled until the solution was fully absorbed. After impregnation theimpregnated support was allowed to stand for two hours at roomtemperature.

Step (2) Precipitation

A solution of 18 ml sodium metasilicate in water was quickly added tothe wet impregnated support. The concentration of the 18 ml sodiummetasilicate solution (SMS) was determined using the following formula:

    1.8×[(1 mole of SMS/mole Na.sub.2 PdCl.sub.4)+(2 moles of SMS/mole HAuCl.sub.4 +(0.02 millimoles SMS/gram of support].

The mixture was swirled intermittently over a period of 15 minutes thenpermitted to stand undisturbed overnight.

Step (3) Reduction

The aqueous phase above the black pellets was treated with an 85%hydrazine hydrate solution. The quantity of hydrazine hydrate used wasdetermined using the formula:

    22.5×[(1 mole N.sub.2 H.sub.4 /mole Na.sub.2 PdCl.sub.4)+(1.5 mole N.sub.2 H.sub.4 /mole HAuCl.sub.4)].

The mixture was gently swirled then allowed to stand undisturbedovernight.

Step (4) Washing

The aqueous phase, which contained a small amount of suspended blacksolids, was decanted and the spheres were washed four times with about50 ml water, decanting after each wash. The catalyst was transferred toa glass column fitted with a stopcock and then washed with further waterat an approximate rate of one liter per 12 hours until the washingsyielded a negative chloride test with silver nitrate solution.

Steps (5)-(7) Drying, Potassium Acetate Loading and Final Drying

The catalyst was dried overnight at 60° C. in a forced air oven, cooled,then impregnated with a solution of the required amount of potassiumacetate in 8.7 ml of water. The mixture was swirled until all the liquidwas absorbed, then the catalyst was again dried at 60° C. on a stainlesssteel screen in a forced-air oven.

Catalyst Test Method and Results

Tests were performed at 7.8 barg and 150° C. on 2.5 g samples of the 5-6mm catalyst pellets, diluted with 30 ml of 1 mm glass beads and loadedinto a stainless steel tube of internal diameter 10-11 mm. The catalystwas activated at 7.8 barg by heating at 160° C. for 3 hours in a streamof nitrogen or helium and then at 150° C. for 10 minutes in stream ofethylene. Acetic acid vapour was then mixed with the ethylene and passedover the catalyst for a period of at least one hour. A mixture of 21%oxygen in helium was gradually added to the feed gas while maintainingthe maximum catalyst bed temperature 150° C. The catalyst hot spot wasmaintained at 150° C. for 6 hours and thereafter allowed to fall as thecatalyst deactivated. The final composition of the reactant mixture wasethylene:acetic acid:oxygen:helium=53.1:10.4:7.7:28.6 and the total gashourly space velocity was 3850 hr⁻¹. The product stream was analysed inthe vapour phase at hourly intervals by means of an on-line gas-liquidchromatograph.

Activity was calculated as grams of vinyl acetate produced per liter ofcatalyst per hour (space time yield, STY) and selectivity was calculatedas the percentage of converged ethylene present in the product. Allfigures quoted are based on the activities and selectivities measured 20hours after full oxygen content was reached.

Alternative Method for Preparing the Catalyst Samples

The following is an alternative, improved method of making the catalyst.

Step (1) Impregnation of the Support

The same method as set forth above was used.

Step (2) Precipitation

The same method as set forth above was used except that sodium hydroxidewas used as the precipitating agent. The amount of sodium hydroxide usedwas determined using the formula:

    1.8×[(2 moles of NaOH/mole Na.sub.2 PdCl.sub.4)+(4 moles of NaOH/mole HAuCl.sub.4)+(0.04 millimoles of NaOH/gram of support)]

Step (3) Reduction

The same method as set forth above was used.

Step (4) Washing

The product of step (3) was washed with water as set forth above.Potassium acetate (5%) however was used instead of water in the columnwashing.

Step (5) Drying

The product of step (4) was dried overnight at 60° C. on a stainlesssteel screen in a forced-air oven.

EXAMPLE 1 Catalyst tests to show the effect of noble metal content oncatalyst activity and selectivity

Catalysts were prepared according to the first general method describedabove. The nobel metal contents were varied to produce a statisticallydesigned set of experiments. The compositions of the catalysts testedwere determined by XRF analysis and are given in Table 1. The targetedpotassium acetate content in each case was 7.0 wt. %, corresponding to2.8 wt. % of potassium. The support as obtained was found to contain 0.4wt. % of potassium giving a total potassium content of 3.2 wt. %. Theweight of catalyst used for each test was varied in the range of 0.5 to2.5 g, to allow catalyst activities and selectivities to be measuredover a range of conversions. The sodium content of the catalysts wasapproximately 0.5% by weight.

The test conditions used were as described above. The total flow ofreactants was such that for a test on 2.5 g of catalyst the GHSV was3850 hr⁻¹. For tests on smaller quantities of catalyst the total flowrate was maintained, giving rise to variations in the GHSV. Activity,selectivity and oxygen conversion measurements were taken after 20 hrson stream and are also given in Table 2.

The variations in activity, selectivity and conversion with catalystcomposition and weight were best fitted to the following expressions:##EQU1##

The abbreviations used in the expressions are Pd=weight percentpalladium in catalyst; Au/Pd=weight ratio of gold to palladium;Cat.Wt=catalyst weight in grams. K=weight percent potassium; STY=spacetime yield in grams of vinyl acetate per liter of catalyst per hour. Thecorrelation coefficients R² indicate a good fit to the data.

In order to determine the effect of the catalyst composition at constantconversion equation 3 was re-arranged to express catalyst weight as afunction of palladium content, gold/palladium ratio and conversion. Arepresentative oxygen conversion of 30% was entered into this function.The catalyst weight terms in equations 1 and 2 were then replaced withthe weight function obtained on re-arranging equation 3 to giveexpressions describing the variation in catalyst activity andselectivity with metals content at constant conversion.

The effect of gold/palladium ratio on activity is shown in FIG. 1, whistthe effect on selectivity is shown in FIG. 2. These curves show thatimproved catalyst activity may be obtained by increasing the palladiumloading of the catalyst and by increasing the gold/palladium ratio up toan optimum of 0.9 to 0.95. Further increasing the gold/palladium ratioabove this reduces the activity of the catalyst.

Selectivity increases are also obtained by increasing the gold/palladiumratio. Finally, as can be sen from FIG. 3, catalysts having a gold topalladium ratio of 0.9 or above produce little or no ethyl acetatebyproduct.

                  TABLE 1                                                         ______________________________________                                        Data Used for Modelling STY, Selectivity and Conversion                       with Palladium Content and Au/Pd Ratio                                        Pd    Au              K     Cat. STY   Sel. O.sub.2 Conv.                     wt %  wt %    Au/Pd   wt %  Wt   (measured at 20 hrs)                         ______________________________________                                        0.58  0.26    0.45    3.3   0.77 801   95.4 12.3                              0.58  0.26    0.45    3.3   2.49 602   94.5 30.0                              0.88  0.42    0.48    3.3   2.50 702   94.9 36.3                              0.91  0.34    0.37    3.2   2.50 654   93.8 33.9                              0.91  0.34    0.37    3.2   2.50 624   94.2 32.2                              0.91  0.34    0.37    3.2   0.76 835   95.2 12.7                              1.01  0.44    0.44    3.1   2.50 727   94.7 35.9                              0.91  0.34    0.37    3.2   1.25 747   94.7 19.0                              1.06  1.21    1.14    3.1   0.85 993   96.2 16.0                              0.58  0.83    1.43    3.2   2.50 592   95.4 28.6                              0.58  0.83    1.43    3.2   0.80 771   96.0 11.4                              0.92  0.83    0.90    3.1   2.46 760   95.4 35.1                              1.00  0.87    0.87    3.1   1.55 939   95.3 28.0                              1.01  0.44    0.44    3.1   0.80 991   95.1 15.7                              0.91  0.34    0.37    3.2   0.50 879   95.8  9.2                              0.91  0.34    0.37    3.2   2.09 709   94.0 30.7                              0.74  0.49    0.66    3.3   2.50 693   94.8 35.7                              0.70  0.29    0.41    3.4   2.50 644   95.2 32.2                              0.88  0.42    0.48    3.3   2.50 691   94.9 36.3                              0.87  1.16    1.33    3.2   0.85 916   96.0 14.5                              0.58  0.26    0.45    3.3   2.48 555   95.5 25.3                              0.92  0.83    0.90    3.1   1.48 937   96.2 26.6                              0.87  1.16    1.33    3.2   0.89 857   96.4 14.0                              ______________________________________                                         3.2 wt % K is approximately equal to 7.0 wt % KOAc                       

We claim:
 1. A process for preparing vinyl acetate which comprisesreacting ethylene with acetic acid in the presence of an oxygencontaining gas at a temperature in the range 100° to 200° C. in thepresence of a shell impregnated catalyst having a productivity atgreater than 661 grams of vinyl acetate per hour per liter of catalystat 150° C. and consisting essentially of:(1) a catalyst support having aparticle diameter from about 3 to about 7 mm and a pore volume of 0.2 to1.5 ml per gram, (2) palladium and gold distributed in the outermost 1.0mm thick layer of the catalyst support particles, and (3) from about 3.5to about 9.5% by weight of potassium acetate wherein the gold topalladium weight ratio in said catalyst is in the range 0.60 to 1.25. 2.A process as claimed in claim 1 wherein the gold to palladium weightratio in the catalyst is in the range 0.7 to 1.05.
 3. A process asclaimed in claim 2 wherein the gold to palladium weight ratio is in therange 0.85 to 1.05.
 4. A process as claimed in claim 1 wherein thepotassium acetate content of the catalyst is 5 to 9% by weight.
 5. Aprocess as claimed in claim 1 wherein the palladium content of thecatalyst is greater than 3.0 grams per liter of catalyst.
 6. A processas claimed in claim 5 wherein the palladium content is greater than 3.9grams per liter of catalyst.
 7. A process as claimed in claim 6 whereinthe palladium content is in the range 3.9 to 6.1 grams per liter ofcatalyst.
 8. A process for preparing vinyl acetate which comprisesreacting ethylene with acetic acid in the presence of an oxygencontaining gas at a temperature in the range 100° to 200° C. in thepresence of a shell impregnated catalyst consisting essentially of:(1) acatalyst support having a particle diameter from about 3 to about 7 mmand a pore volume of 0.2 to 1.5 ml per gram, (2) palladium and golddistributed in the outermost 1.0 mm thick layer of the catalyst supportparticles, and (3) from about 3.5 to about 9.5% by weight of potassiumacetate wherein the gold to palladium weight ratio is greater than orequal to about 0.9.
 9. A process as claimed in claim 8 wherein thepotassium acetate content of the catalyst is 5 to 9% by weight.
 10. Aprocess as claimed in claim 8 wherein the palladium content of thecatalyst is greater than 3.0 grams per liter of catalyst.
 11. A processas claimed in claim 10 wherein the palladium content is greater than 3.9grams per liter of catalyst.
 12. A process as claimed in claim 11wherein the palladium content is in the range 3.9 to 6.1 grams per litercatalyst.